This invention relates to anesthesia machines, and in particular, to anesthesia machines which are easily transportable.
Anesthesia machines are well known in the art for use in anesthetizing humans as well as other animals for surgical purposes. Virtually all prior art anesthetic machines are intended to be fixedly mounted in a stable environment, such as in a hospital operating room. This is usually required by the design of the machine and especially the vaporizer in which the liquid anesthetic agent is contained. Should the anesthetic machine be jostled, or its attitude be changed such as by tilting or the like, control of the rate of evaporation of the liquid anesthetic is affected and, in extreme cases of tilting or inversion, liquid agent may be directly introduced into the various passageways of the anesthesia machine such that the patient could be directly exposed to dangerous levels of the anesthetic agent. Such a circumstance could result in disastrous consequences.
When vaporizers are agitated or tipped more than 45 degrees from the vertical, liquid anesthetic is often discharged into the control mechanisms, obstructing valves. This results in the delivery of a large bolus of saturated vapor to the patient which could results in overdose. Other hazards have been reported involving calibrated vaporizers from several manufacturers. Displacement of liquid from a vaporizer to its outlet tubing can cause patient overdose, because not only the vaporizer flow, but total carrier gas flow then vaporizes the liquid anesthetic agent in an uncontrolled manner. Other known hazards include the possibility of using an incorrect agent in a vaporizer and some anesthesia machines are not compatible with MRI environments. They create artifacts that cause monitoring errors when used in these settings.
In addition, control of the delivery of anesthetic agents to patients when using both permanent and portable anesthetic machines can be improved upon. The two methods for controlling drug administration are open and closed-loop control. In open loop control, depth of anesthesia is regulated by adjusting a dial on the vaporizer to increase or decrease the concentration of the anesthetic gas delivered to a patient. This is the prevalent method today. Depth of anesthesia is observed by monitoring the patient's vital signs, using technologies that measure the patient's brain waves (electro-encephalogram), the heart's electric signals (electro-cardiogram), blood pressure and respiratory rate. Recent advances in closed-loop control of anesthesia have only focused on how to use these signals to control drug delivery.
Researchers caution that the results produced by controllers that depend on patient vital signs to regulate drug delivery to achieve depth of anesthesia, are unreliable and pose challenges because of the differences between patients. These patient dissimilarities have been attributed to genetic, enzymatic and metabolic differences. Recent articles attest that while measured values like blood pressure or muscular activity are direct indicators of the controlled variable, others like EEG are not. It is thus difficult to control the depth of anesthesia of a patient when a direct measurement of the drug effect is not possible.
No precedence exists for the application of closed-loop control to inhalation anesthetic vaporizers.
As is known, the vapor pressure of a gas is affected by variables such as temperature and pressure. As is also known, as the anesthetic in the vaporizer evaporates, the vaporizer chamber will cool, thus affecting the vapor pressure of the anesthetic agent and the amount of anesthetic agent that is entrained by the carrier gas. To maintain a proper amount of anesthetic agent, the flow rate of the carrier gas is typically adjusted at the inlet of the carrier gases to the anesthetic machine, and such adjustments are made manually. As can be appreciated, adjusting the flow rate of the individual gases which comprise the carrier gas can affect the overall composition of the carrier gas (i.e., the percentage of carrier gas made up of O2, N2O, and air. The manual modulation of gas flow rates to compensate for decline in anesthetic concentration due to cooling, or changes in barometric pressure, to maintain anesthetic depth is a tedious and unreliable process, because of the time difference between the observed physiologic signal and the physician response. The challenges of mechanical control of vaporizer performance, necessitates an evolution to electromechanical control, integrating a degree of intelligence not found in any vaporizer in commercial existence, to achieve consistent precision and reliability. Such a vaporizer will pave the way for a new class of field anesthesia machines.
While the hazards of existing vaporizers are known and safety recommendations have been published only a partial solution has been attempted by manufacturers, which is aimed at their traditional hospital based business. This includes precision vaporizers that can only be used with one agent. Precision vaporizers by design are agent-specific and utilize indexed funnels to prevent accidental filling with the wrong agent. This means that healthcare providers must purchase multiple vaporizers (one per agent), which is cost prohibitive in an era of cost containments and reimbursement cuts. Existing anesthesia machines which incorporate traditional vaporizers are not suitable for ambulatory applications because of their sensitivity to orientation, shock and vibration, as would be encountered in the field, and are worsened by their dependence on availability of compressed gas and electricity to operate. In addition, they are bulky, heavy, and complex to setup and maintain, and require tremendous logistical overhead to transport. There is no transportable anesthesia machine that is designed specifically for field applications, and in particular non-traditional surgical environments.
Military and civilian doctors have expressed a need for transportable anesthesia machines with reduced logistics footprint, to facilitate far-forward surgical care in combat and non-traditional operating environments. Unlike previous anesthesia machines, it would be one-man transportable, reliable, simple, tippable, insensitive to shock and vibration, and vagaries of weather, does not require compressed gas, easy to setup, use and maintain, and compatible with standard off-the-shelf physiologic monitors and portable ventilators. Transportable anesthesia machines would also be of use to disaster care organizations and humanitarian care organizations, such as FEMA (Federal Emergency Management Agency), Doctors Without Borders, Wings Of Hope, Red Cross, etc., for mass civilian casualty care and surgery in third world settings.
Such anesthesia machines will help to perform life resuscitative surgery in the battlefield, “to extend the ‘golden hour’ for treatment, in order to improve survival rate from life threatening injuries, including those from improvised explosives, and minimize morbidity after other life-threatening injuries”. These technologies will improve the first responder's capability to provide effective treatment more rapidly and as close to the place of the injury as possible. Furthermore, they will be of immense utility during peace keeping and post-war or post-disaster reconstructive initiatives around the world. “Without the appropriate equipment, management of the seriously injured patient is impossible for even the most competent anesthetist.”
Today's medical equipment and in particular general anesthesia machines are not suitable for the delivery of critical medical care or emergency surgery in less than ideal terrains or environments. The answer is not expensive medical devices but practical and reliable equipment, designed for the surgical conditions encountered in the field. It has long been recognized by physicians practicing in the field that the essentials for safe anesthesia in any situation does not depend on expensive equipment but safe anesthesia induction, a secure airway, adequate tissue oxygenation, appropriate monitoring and recovery. This is reinforced by the fact that in many parts of the world resources are scarce, and even a regular supply of compressed gases such as nitrous oxide and oxygen is not possible. This poses a major challenge for anesthesiologists working in these environments.
Portable anesthesia machines could be used in the field, or in combat-surport hospitals, to stabilize patients prior to transporting the patients to full service hospitals. Surgical care in the field is limited logistically by the size, weight. complexity and sensitivity of traditional anesthesia machines to environmental conditions. Currently, field hospitals must carefully transport, and then set up, these anesthesia machines. Such a system is not feasible when the machine is to be used at a locale for only a short period of time. In addition, small clinics serving patients in third world countries typically cannot afford an anesthesia machine. Hence it would be desirable to have such a machine which is easily transportable to allow two or more clinics to share one machine, and to move the anesthetic machine between the clinics as needed. Because such clinics generally cannot afford, and hence do not have, anesthesia machines, any patient requiring even minor surgery must be transported at great difficulty to the nearest city having a hospital or clinic with an anesthetic machine. It would be desirable to provide an anesthesia machine which could be used in such situations.
An historical approach to overcoming the shortage or absence of compressed gas is the use of a draw over vaporizer, which works in an open circuit mode, similar to the Oxford Miniature Vaporizer. Draw over systems are designed to provide anesthesia without requiring a supply of compressed gases. Atmospheric air is used as the main carrier gas and is drawn by the patient's respiratory effort through the vaporizer, containing the volatile anesthetic agent. The mixture is then inhaled by the patient through a non-rebreathing valve. These types of vaporizers typically have low internal resistance, and the volume of air passing through the vaporizer is determined by the patient's tidal volume (the volume of air in a single breath) and the respiratory rate.
The machines now used for combat casualty care, such as the NarcomedM, Ohmeda 885A and the Oceanic Magellan are too big and complex, require compressed gas and electricity, plus battery backup, extensive setup time, and logistical overhead to deploy. The NarcomedM from Datex Ohmeda for example weighs 163 pounds and requires two men to transport.
For decades, there has been a continual increase in the number of surgeries performed on an outpatient basis and in the field, yet anesthesia machines have not advanced to address the unique requirements of inhalation anesthesia induction in these environments, such as reduced footprint and weight, and reliability in extreme environmental conditions, outside of the traditional operating rooms, from private physician clinics to the far-forward location of the battlefield and third world settings. They are bulky and cannot be transported by a single medic or first responder on foot, and depend on an agent specific vaporizer which necessitates the transport of multiple vaporizers, for different anesthetic agents; escalating the weight penalty and logistics burden. Furthermore, they require infrastructure that are often not available in the environment. Because of their sensitivity to orientation and vibration, they cannot be used without recalibration if tipped or jolted, making them unsuitable for surgical applications in a theater that is susceptible to vibration, such as a war zone, or moving medical transport vehicle.
In the asymmetric warfare of the 21st century where soldiers can be attacked and wounded anywhere, medical teams must be agile and able to work as close as possible to the battlefield. The potential escalation of casualties in this type of warfare due to weapons of mass destruction and improvised explosive devices, in unexpected areas, including alleys, underscores a need for medical personnel to be able to perform life resuscitative surgery anywhere. This is reinforced by the likelihood that a medical evacuation vehicle while on-route to retrieve wounded soldiers, may be destroyed and thus fails to deliver medical assistance within the golden hour.